On New Strata in the Torgashinsky District of the Eastern Sayan

(Preliminary Communication)

K. V. Radugin

1. In the summers of 1963 and 1964 the author made short traverses in the Eastern Sayan, along the right slope of the Bazaikha valley near the mouths of its left tributaries, the Mokhovaya and the Kaltat. This sector of the Torgashinsky district of the Eastern Sayan has long attracted attention. It is located near Krasnoyarsk. Many geologists therefore visited it from the time of the discovery and study of the first finds of trilobites (Schmidt, E. V. Lermontova) and archaeocyathids (A. G. Vologdin, 1931). The works of these geologists are well known (V. A. Obruchev, A. G. Vologdin, I. K. Bazhenov, A. N. Churakov, Yu. A. Kuznetsov, A. A. Predtechensky, I. I. Koptev, S. I. Makarov, I. T. Zhuravleva, A. Yu. Rozanov, K. A. Konyushkov, L. N. Repina, M. A. Semikhatov, V. V. Khomentovsky, and others). A particularly full history of their investigations was given by A. A. Predtechensky (unfortunately, in an unpublished work) and V. A. Obruchev (in his well-known five-volume History of the Geological Exploration of Siberia). More recently, reviews of the investigations of the same area were given by L. N. Repina, M. A. Semikhatov, and V. V. Khomentovsky (1960). All these reviews retain their significance, and there is no need to repeat them. However, we shall emphasize the principal importance, for the present tectono-stratigraphic communication, of the works of the following geologists and paleontologists: V. A. Obruchev (1917), A. G. Vologdin (1931), I. K. Bazhenov (1937), I. T. Zhuravleva (1960), I. I. Koptev (1961), as well as the works of a group of geologists of the Geological Institute of the Academy of Sciences of the USSR—L. N. Repina, M. A. Semikhatov, and V. V. Khomentovsky (1960)—and of the geologists of the Krasnoyarsk Geological Administration headed by S. I. Makarov. We should also note an interesting find of rare archaeocyathids below the Torgashinsky Formation by K. I. Konyushkov and, somewhat later, by A. Yu. Rozanov (according to their unpublished communication, in the Kaltat beds). In the following communication, in addition, some of our observations made in 1936 and later are taken into account.

It would seem that the section of interest to us, on the right slope of the Bazaikha valley near the mouths of the Kaltat and Mokhovaya, investigated by so many geologists, has been studied in detail (the result of this study is presented below in Table 1). And yet new elements have been found in this section. Before describing them, we shall present the result of the preceding investigations in a summary scheme (Table 1).

Despite the small thickness of the Bazaikha beds (about 75 m), they were subdivided into three parts: the upper part with trilobites of the Kameshkovsky complex, the middle with trilobites, and the lower without trilobite-

part with archaeocyaths of the Bazaikha complex¹. It should be noted that just recently the results of the study of the section opposite the mouth of the Kaltat apparently proved not entirely satisfactory, since at the 1965 conference on the stratigraphy of the Cambrian and Precambrian of Siberia the Cambrian section of the Torgashinsky district was not considered. And indeed this section, as our observations have shown, must be refined and supplemented. Using these new and our previous observations, we can present the same section in the following form, beginning from the top (Table 2).

Table 1

Scheme of the stratigraphic column of the section on the right slope of the Bazaikha valley below Kaltat

1. At the top lies the Torgashinsky limestone, underlain by the so-called Bazaikha beds (or horizons), whose thickness, according to the determination of I. K. Bazhenov and M. P. Nagorsky (1937), is 80 m (according to I. I. Koptev, 75 m). A bed-by-bed description of them was given by the same investigators, who distinguished among the archaeocyaths from the Bazaikha beds opposite the Kaltat those synchronous with them and older ones, contained in fragments of light-colored limestone or fine-crystalline marble. Both the former and the latter archaeocyaths, identified by P. S. Krasnopeeva, were assigned by her to the Kameshki complex. We confirmed the presence of the older archaeocyaths in the fragments soon after 1937, when the Bazaikha beds were traced for a short distance along strike to the point of their disappearance. At that time, at the base of the Bazaikha beds, inclusions of large boulders—blocks of archaeocyath limestone or marble—were discovered, reaching up to 5 m in height and along the bedding. Some at first assumed that these were not boulders, but bioherms among near-reef and inter-reef deposits. However, our observations showed that they are indeed large isometric blocks (or boulders) that had undergone karstification before burial in the surrounding beds. By the time of this karstification, the archaeocyath massive limestone composing the blocks had undergone some degree of alteration; it already contained cracks and calcite veinlets along them.

From these observations, the correct conclusion was drawn at the time that some carbonate unit with archaeocyaths had been destroyed, producing nearby large Lower Cambrian rocks. However, at that time the question

that where this unit crops out, I did not decide. Observations in 1963 revealed it beneath the Bazaikha beds for several hundred meters below the mouth of the Kaltat, where it has in part been preserved and is observed in the cliffs.

Table 2

Scheme of the stratigraphic column of part of the right slope of the Bazaikha valley near the mouths of the Kaltat and Mokhovaya

There it is represented by massive, nearly white, unstratified reefogenic archaeocyath limestones, or by fine-crystalline marbles, typical of the Lower Cambrian. Among the archaeocyaths in the field the following genera were found: Bachathocyathus, Archaeolynthus, Ajacicyathus, Kocyinocyathus, Dokidocyathus, irregular archaeocyaths, presumably Syringocnema, and others. In addition, there are epiphytons, Razumovskia, and their associated forms. In places brown ironstone occurs in the field of the unit being described.

The archaeocyath-bearing member described (or, more probably, a thin unit poorly preserved from erosion) was evidently not previously taken into account in the sections described, since the Kaltat unit was shown immediately beneath the Torgashino limestone and the Bazaikha beds (or beneath the so-called Bazaikha “horizon”). The latter archaeocyath-bearing unit, which we shall provisionally call the lower pulsation suite, presumably lies with a break. In turn, it is overlain with a break by another sub-suite (the upper pulsation suite, or sub-suite), not previously described, exposed along one of the ridges of the right slope of the Bazaikha valley and underlying the Bazaikha beds. On both sides of this ridge it cannot be traced far.

being eroded before the ingression of the Torgashino sea (of “Bazaikha” time).

What, then, is this second pre-Bazaikha, thin sequence (its thickness is tens of meters) composed of? In its lower part it is characterized by sandy-clayey reddish-lilac basal beds. Their thickness is apparently no more than several meters. The upper part of the same sequence is lithologically uniform, represented by grayish platy fine-crystalline limestones—marbles, interbedded with layers that weather brown and are enriched in sandy-clayey, terrigenous material. No fossils, apart from sponges, have been found. In places there are accumulations of hematite. It is characteristic that, beneath the lilac basal beds, the underlying archaeocyathid biohermal structures bear clear traces of weathering under continental conditions: they were karstified along fractures, and then, before the ingression of the sea, washed-in variegated sandy-clayey deposits accumulated in these widened fractures and sedimentary veinlets were formed.

In connection with these traces of a break, apparently related to uplifts, and taking into account the subsequent subsidence and new uplift (i.e., pulsations), we can only regard the described pre-Bazaikha beds as a pulsational suite, despite its small thickness. Such a thickness is explained by erosion that occurred before the accumulation of the overlying transgressive series of deposits, which as a whole may be considered the Torgashino geological formation (or pulsational suite), within it distinguishing the basal variegated beds (the Bazaikha beds, also called the Bazaikha “horizon” of local significance).

It remains to supplement the characterization of the section described and to note the poorly exposed beds lying beneath the archaeocyathid lower pulsational suite and probably included in the latter as its basal part. These are chiefly grayish sandy-clayey beds. Their composition and stratigraphic affiliation are not entirely clear. Earlier, apparently, all or nearly all these beds were assigned to the Kaltat sequence, which is hardly correct. Beneath them lies a characteristic packet of light dolomites, and still lower occurs the grayish Kaltat sequence of sandstones, calcareous-clayey shales, and fine-crystalline epiphyton marbles.

In the described section, located opposite the mouth of the Kaltat, there is still another sequence. It was overlooked, but we shall note it as essentially important for understanding the tectono-stratigraphic relations in the Torgashino district. This sequence, distinguished by the author, was correlated by I. I. Koptev (1961) with the Ungut sequence. It is composed mainly of dolomites of the Mokhovaya syncline. These dolomites are exposed in the rocks of the right slope of the Bazaikha valley below the Kaltat and above Red Cliff, in the area of the mouth of the Mokhovaya. They have also been assigned to the provisionally distinguished Krasnokamensk dolomite sequence.

The lithological aspect of its dolomites is as follows. They are light in color (from white to light gray or grayish), in places pinkish or light red, fine- or very fine-crystalline, everywhere or almost everywhere unbedded, without signs of algal structure, dense, strongly fractured, locally brecciated and silicified. The degree of silicification varies sharply from zero to 100 percent, increasing greatly in zones of thickened fracturing and crushing. Silicification appears either in the form of scattered small irregular inclusions, or as concentrated small veinlets forming a kind of stockwork, or as rather large, more often vein- ԥ

different or stock-like masses. Two (if not more) stages of silicification are observed, which is established from the intersection of veinlets of microquartzite composition. The age of the silicification has not yet been established. Since silicification is developed almost exclusively in the dolomites described, it might be supposed that in the main the processes of their silicification were completed before deposition of the overlying archaeocyathid sequence of the Lower Cambrian, and that only additional silicification is connected with the dikes (diabase, microsyenite, etc.) cutting this sequence. The age of the dikes is not younger than Devonian. We may also accept this upper limit for the silicification of the dolomites of the Krasnokamenskaya sequence.

In them there are traces of still another process connected with the break before deposition of the archaeocyathid sequence (the lower pulsational suite). During the break, deep denudation took place, and the dolomites of the Krasnokamenskaya sequence were preserved only in the Mokhovaya syncline, where on top they bear clear traces of an ancient weathering crust. It is expressed in a light-red and pink uneven coloration of the dolomites and in the formation of small karst cavities filled with darker reddish-gray and more fine-grained dolomite than the dolomite of the walls of these cavities. Also characteristic is the greater uniformity of the fine grain of the karst illuvial veinlets. There is no doubt that their material is the result of comparatively fine comminution of dolomite crystals during physical weathering. In places, in sedimentary dolomitic karst or cave veinlets, bedding is visible, directed almost along the walls of the karstified cavities and expressed by alternation in the color of laminae, now lighter, now darker.

The sedimentary cave veinlets were cut by tectonic fractures of several systems; then micro-displacements occurred along them, and hydrotherms deposited thin veinlets of microquartzite composition and red coloration. This proves that at least one phase of silicification of the dolomites is younger than the formation of the above-described sedimentary veinlets of the ancient karst weathering crust.

The tectonics of the Krasnokamenskaya sequence, because of the absence of bedding in the dolomites, is unclear. Nevertheless, the structure is taken to be synclinal, since beneath it we have established the conglomerates of Krasny Kamen and Konglomeratovyi Log (Belousov and Radugin, 1956), which indicate the position of the footwall of the dolomites and have been found in the NW limb of the Mokhovaya syncline. In the SE limb of the syncline, coarse-clastic rocks also occur, although they are poorly developed there. The thickness of the Krasnokamenskaya sequence is considerable, but outside the Mokhovaya syncline it has not been precisely established, since next to it it was destroyed by denudation before deposition of the lower pulsational suite. Beds of the latter were observed by us in the axial zone of the NE Mokhovaya syncline, where their strike is az. 350, and the dip is at an angle of 50° to the east. Above these beds lies the Torgashinskaya Formation (the Bazai variegated basaltic beds of it¹).

Thus, the Krasnokamenskaya sequence was preserved from erosion only in the Mokhovaya syncline. If this is so, then it proves a phase of folding that took place after deposition of this sequence. In this connection it is necessary once again to note how unreliable are hasty conclusions about conformable relationships of sequences, obtained on the basis of study of only separate sections and not supported by detailed mapping of geological boundaries.

The relationships of the Krasnokamenskaya sequence with the Kaltatskaya, unfortunately, are still not entirely clear. However, the fact that the former composes a syn-

...syncline and lies next to the Kaltat, points quite definitely to a younger age for the Kaltat Formation in comparison with the Krasnokamensk Formation.

1. Thus, the observations presented above and their analysis with the aid of a detailed geological map, and with allowance for elements of tectonics, provide a basis for the following refinement of the stratigraphic section (Table 2), as compared with that given in Table 1.

Let us recall that, within the Bazaikha complex of archaeocyaths established in the Bazaikha basal beds of the Torgashinsky geological formation, forms were identified by which the lower of these beds were correlated with the Upper Keniadian beds of the Siberian Platform. If this is taken into account, then the Lower Keniadian and Sunagin beds of the same platform may correspond to the upper and lower pulsational suites. This is possibly not contradicted by the archaeocyath composition of the latter. To the same lower levels of the Lower Cambrian evidently belong the poorly exposed gray-colored sandy-clayey deposits, about 70–90 m thick, which are most likely the basal member of the lower pulsational suite. As for the Krasnokamensk pulsational suite, or geological formation, and the Kaltat unit, they are separated from the overlying pulsational suites by a phase of folding and denudation; therefore, their assignment to the Cambrian cannot at present be considered justified. True, according to K. I. Konyushkov and Yu. A. Rozanov, rare archaeocyaths are now also present in the Kaltat unit. If only they have indeed been found in it, and not in the overlying, poorly exposed member of gray-colored sandy deposits, then these finds are of great interest as finds of Precambrian (?) archaeocyaths. Of course, in this we proceed from the concept of the lower boundary of the Cambrian with the Proterozoic according to which this boundary cannot at present, without revision of the stratotype of the Lower Cambrian in England, be lowered below the Olenellus zone (the Holmia subzone, correlated with the Judomian beds of the Siberian Platform).

REFERENCES

1. I. K. Bazhenov, M. P. Nagorskii. Geology of the Krasnoyarsk area. Materials on the Geology of Krasnoyarsk Krai, issue 1. Tomsk, 1937.

2. A. F. Belousov, K. Radugin. On the relationship of the Torgashinsky limestone to the oldest units of the Krasnoyarsk Range. Transactions of the Mining-Geological Institute, WSFAN USSR, issue 17, 1956.

3. A. G. Vologdin. Archaeocyaths of Siberia, issue 1, published by GTU. Moscow, 1931.

4. I. T. Zhuravleva. Archaeocyaths of the Siberian Platform. Publishing House of the Academy of Sciences of the USSR. Moscow, 1960.

5. I. T. Zhuravleva, A. Yu. Rozanov. Age and conditions of formation of the Bazaikha and Biryusa limestones. Geology and Geophysics, No. 3, 1962.

6. I. I. Koptev. New data on the age of the Torgashinsky limestones. News of Higher Educational Institutions—Geology and Exploration, No. 4, 1961.

7. V. A. Obruchev. On the Torgashinsky limestone and the Yenisei suite. Geological Herald, 3, No. 1–6, 1917.

8. L. N. Repina, M. A. Semikhatov, V. V. Khomentovskii. Regional Stratigraphy of the USSR, issue 4, Moscow, 1960.

1. In this area S. I. Makarov mapped a disjunctive fault. But he was evidently mistaken, since he did not change the stratigraphic sequence of the units. ↩↩